Method and device for measuring the concentration of soot particles in an exhaust gas, in particular from an internal combustion engine

ABSTRACT

The present invention relates to a method and a sensor ( 1 ) for collecting soot in an exhaust stream from an internal combustion engine by means of a resistive type planar probe positioned in the gas stream and having at least one face provided with electrodes connected to an electronic device for measuring soot concentration in the gas stream. The gas stream upstream from the planar probe is deflected by means of a protection shield ( 3 ) completely covering the planar probe and having a single soot-collection window ( 5 ) arranged in a wall of the protection shield ( 3 ) that is situated downstream from the planar probe in the gas stream, in such a manner as to:
         protect the planar probe from a direct frontal impact of soot and any debris contained in the gas stream; and   generate a turbulent flow and reduce the speed of the gas stream downstream from the planar probe so as to select soot particles as a function of their size.

The present invention relates to the field of collecting particles in gas streams and measuring their concentration, and in particular measuring the concentration of particles in the exhaust gas of internal combustion engines.

More particularly, the invention relates to a method of collecting particles in a gas stream. The invention also relates to a soot sensor specially designed for performing the method.

The invention applies most particularly to measuring the concentration of carbon particles, known as soot, in the exhaust gas from internal combustion engines, more particularly for the purpose of qualifying the state of particle filters within a motor vehicle exhaust circuit.

The present invention may thus be performed equally well in a position downstream from the particle filter in order to monitor its integrity, or upstream therefrom in order to optimize regeneration cycles of the particle filter and thereby minimize fuel consumption. The invention also seeks to improve the robustness of a particle filter, and to enable it to be dimensioned as closely as possible to the bare minimum.

In the field of the invention, sensors are already known, e.g. from patent application EP 2 228 522, in particular for use in the automobile field, for the purpose of measuring soot concentration within an exhaust stream in particular downstream from the particle filter in the vehicle exhaust circuit in order to diagnose the state of the particle filter. Such sensors are constituted in particular by resistive type sensors having two conductive electrodes spaced apart from each other, with variation in electrical resistance between them being measured as the sensor becomes progressively clogged as a result of soot becoming deposited between said electrodes. Resistance measurement is performed automatically in application of a determined cycle under the control of the electronic control unit of the vehicle or of the sensor, which unit is connected to the terminals of the electrodes of the sensor. The electronic control unit then calculates soot concentration from the measured resistance in order to qualify the state of the particle filter and trigger a filter replacement warning, where appropriate.

The resistance measured across the terminals of the electrodes of the sensor varies by decreasing until it saturates with the progressively increasing quantity of soot deposited on the sensor. This decreasing variation is the result of carbon connection bridges becoming established between the electrodes of the sensor as a result of soot being deposited thereon when the sensor is located in the exhaust stream from the vehicle while the particle filter is deflective.

When the resistance curve as measured across the terminals of the sensor reaches a horizontal asymptote, the sensor is said to be saturated. It is then necessary to regenerate it in order to perform new measurement cycles. This operation is performed by means of a heater resistor that serves to burn off the soot deposited between the electrodes by heating the surface of the sensor and the soot, thus enabling the sensor to be regenerated for new measurement cycles.

Although they appear to be very simple in operation, such soot sensors, which may equally well be of the capacitive type instead of the resistive type, are nevertheless subjected to a large number of constraints that can disturb their integrity and their operation.

In particular, the thermal stresses in the exhaust stream of an internal combustion engine are very great, given that the temperature of the exhaust gas stream may be as high as 900° C. to 1000° C. The sensor and its components must thus be capable of withstanding such temperatures and also be capable of performing measurements that are accurate and reproducible, which is a challenge at such ambient temperatures in a gas stream flowing at high speed, which speed may be of the order of several tens to one hundred meters per second.

Furthermore, the temperatures and the speeds of the exhaust stream also have a great influence on the regeneration properties of the sensor once it has saturated, implying in particular a very high temperature, higher than 550° C., for heating by the regeneration resistor of the sensor in order to burn off the soot, and in certain circumstances that can be found to be particularly difficult.

Soot sensors are also subjected to major mechanical stresses because of impacts from soot or metal and/or ceramic debris coming from a deflective particle filter and striking the measurement surface of the sensor. Although these particles are of very small size and very small weight, they come into contact with the sensor at a speed close to the speed of the exhaust gas stream, and thus with very high levels of kinetic energy. Sensors must therefore also be capable of withstanding such mechanical stresses.

Finally, the response of a soot sensor is also determined by the uniformity of the deposit of soot thereon, and consequently on the sensor being positioned to collect soot as uniformly as possible, and it is also of great importance to target soot in the exhaust stream, i.e. to select particles of a size less than 500 nanometers (nm).

The various presently known soot sensors are found to be unsatisfactory for the most part. Either they are too fragile faced with the temperature and mechanical constraints associated with their use, or else they are not very accurate, given the difficulties of regenerating the sensor and the difficulty of performing measurements as a result of soot becoming deposited in non-uniform manner on the electrodes.

The object of the present invention is to produce a solution for collecting soot and for measuring soot concentration in an exhaust stream, which solution is improved and suffers much less or not at all from the above-described problems of existing sensors.

To this end, the present invention firstly provides a method of collecting soot in a gas stream, in particular an exhaust stream from an internal combustion engine, by means of a planar probe, preferably of the resistive or capacitive type, the probe being positioned in the gas stream and having at least one face provided with electrodes and an electrical resistor connected to an electronic device for measuring soot concentration in the gas stream. According to the method of the invention, the gas stream upstream from the planar probe is deflected by means of a protection shield completely covering the planar probe and including an open soot-collection window arranged in a surface of the protection shield that is situated downstream from the planar probe in the gas stream, in such a manner as to:

-   -   protect the probe from the direct frontal gas stream of soot and         any debris that might be contained in the gas stream; and     -   generate a turbulent flow and reduce the speed of the gas stream         downstream from the planar probe so as to select particles         depending on their size;     -   thereby obtaining mechanical protection for the planar probe and         uniform deposition of soot on said planar probe.

Furthermore, the invention also provides a soot sensor for measuring the concentration of soot in a gas stream, in particular an exhaust gas stream from an internal combustion engine. The sensor comprises in known manner at least one resistive or capacitive type planar probe made up of a plane substrate of insulating material including:

-   -   on a first face at least two conductive electrodes faced apart         from each other by a determined distance and connected to         respective electrical connection tracks; and     -   at least one regeneration resistor for regenerating the         electrodes and for measuring the temperature of the gas stream,         said resistor being connected at its terminals to two electrical         connection tracks.

According to the invention, the proposed soot sensor includes a protection shield completely covering the planar probe over all of its faces and including an open soot-collection window arranged in a surface of the protection shield that is situated downstream from the planar probe in the gas stream, the collection window enabling soot to be deposited on at least one collection zone of the first face of the probe when the sensor is positioned in a gas stream.

Various other characteristics appear from the following description made with reference to the accompanying drawings, which show embodiments of the invention as non-limiting examples.

In the accompanying figures:

FIG. 1 is a face view of a soot sensor of the invention fitted with a protection shield in a preferred embodiment;

FIG. 2 shows the FIG. 1 sensor in longitudinal section on its axis X-X′ on a plane perpendicular to the view of FIG. 1;

FIG. 3 shows the sensor of FIGS. 1 and 2 in back view relative to the view of FIG. 1;

FIG. 4 is a cross-section view on a plane C-C of FIG. 2;

FIGS. 5A and 5B show the two faces of a planar probe for measuring soot concentration in a gas stream as implemented in a soot sensor in accordance with the invention;

FIGS. 6A to 6D show different variant embodiments of the protection shield for a soot sensor of the kind shown in FIGS. 1 to 4;

FIG. 7 is a diagram showing the potential angular opening angle of the collection window for a cylindrical protection shield as shown in particular in FIGS. 6A to 6D;

FIG. 8 shows various simulations of the soot sensor of the invention in use in a gas stream such as an exhaust stream from an internal combustion engine, showing the effectiveness with which the gas stream is disturbed and particles are collected as a function of mean particle size;

FIGS. 9A and 9B show the variations in soot concentration as measured with a soot sensor in accordance with the invention, respectively as a function of particle size at a constant speed for the gas stream, and as a function of stream speed for different sizes of particles under consideration;

FIG. 10A is a face view of another embodiment of a soot sensor of the invention; and

FIG. 10B is a cross-section view on a plane B-B of FIG. 10A.

The present invention relates to a method of collecting soot in a gas stream, and also to a soot sensor for performing the method.

The method and the sensor proposed by the invention seek to make it possible in reproducible and accurate manner to measure the concentration of soot, i.e. of carbon particles, present in a gas stream.

A particular application of the soot sensor and method of the invention relates to measuring soot concentration in the exhaust gas circuit of vehicle internal combustion engines, downstream or upstream from particle filters, in order to verify the effectiveness and the performance of such filters and/or to diagnose their state and anticipate regenerating or changing them and/or to measure the weight of soot stored in the particle filter, in order to trigger regeneration of the filter. The description below is given with reference to such an application, taking into consideration an exhaust gas stream and an exhaust pipe of a motor vehicle fitted with a particle filter, and with the state of the particle filter being managed by an electronic control unit that also performs other functions.

FIGS. 1 to 4 show a sensor of the invention in a first preferred embodiment.

The soot sensor 1 of the invention comprises a support body 2 and a protection shield 3 covering a resistive planar probe 6 as shown more particularly in FIGS. 5A and 5B, which probe is fastened via its base to the support body 2. The support body 2 constitutes a mounting cylinder for mounting the planar probe 6 and for the protection shield 3. In particular, it receives electrical connection members for connecting the planar probe 6 to the electronic control unit of the vehicle. It also serves as a mounting body for mounting the soot sensor 1 in the exhaust circuit of the vehicle, in a position upstream or downstream from the particle filter, e.g. using a screw-and-nut system or any other mounting member commonly used for mounting sensors in gas ducts.

In the invention, the protection shield 3 has a soot collection window 5. The collection window 5 is cut out in the wall of the protection shield 3 so as to enable soot to be collected and deposited on the planar probe 6 arranged inside the protection shield 3 and when the soot sensor 1 is positioned in a gaseous exhaust stream.

In preferred manner, the support body 2 and the protection shield 3 are made of a metal material such as Inconel 600 and, as shown in the accompanying figures, they are generally in the form of a right cylindrical tube. Such a right cylindrical shape presents the advantage of being easy to make at low cost, while also providing an internal space for comfortably housing the planar probe 6, with the convex outer surface of the cylindrical shape facilitating the flow of the exhaust gas stream without too much loss of head. Nevertheless, such a cylindrical shape is not itself essential to making the soot sensor 1 and it is easy to envisage any other optionally tubular shape suitable for protecting the planar probe and generating a disturbance in the flow of the gas stream downstream from the probe. A right tubular shape presenting a longitudinal axis and of cross-section that is of a closed regular geometrical shape inscribed in a circle centered on the longitudinal axis along which the shield extends is nevertheless preferred for making the protection shield 3.

In preferred manner, the collection window 5 formed in the protection shield 3 is subdivided by reinforcing crosspieces 52 into a plurality of openings 51 that are identical in shape and area. The reinforcing crosspieces 52 do not perform any role concerning the soot collection performance of the soot sensor 1 in comparison with a collection window 5 not having them. However, the crosspieces 52 serve to reinforce the peripheral surface of the collection window 5 and to prevent the protection shield 3 being flattened or dented in the vicinity of its collection window 5. Such flattening can unfortunately take place in the absence of such reinforcing crosspieces while the soot sensor 1 is being handled, in particular while it is being put into place or removed from an exhaust duct.

As can be seen more particularly in FIGS. 6A to 6D and 10A, 10B, the exact shape of the collection window 5 in the protection shield 3 may vary, in particular concerning the number of openings 51 defined by the reinforcing crosspieces 52.

As in the example of FIGS. 1 to 4 and as shown in FIG. 6A, the collection window 5 may be subdivided into four rectangular openings 51. In a variant, as shown in FIGS. 6B and 6C, the collection window 5 may equally well be subdivided merely into two rectangular openings 51. Also, as shown in FIG. 6D, the collection window 5 in the protection shield 3 may be subdivided into a still greater number of openings 51, e.g. eight openings 51. In a second preferred embodiment, the collection window 5 is subdivided into three windows 51 of oblong shape that are parallel to one another and that are oriented parallel to the longitudinal axis X-X′ of the soot sensor.

The collection window 5 and its openings 51 may be made by being cut out (using a laser or a water jet, for example) in the cylindrical wall of the protection shield 3, or indeed by stamping, punching, or molding the protection shield 3. It is thus possible to select the precise shapes of the collection window 5 and of its openings 51.

As shown in FIG. 7, the width l of the collection window 5 may be such that, relative to a cylindrical protection shield of circular cross-section, said collection window 5 covers an angular sector β that preferably lies in the range 45° to 180° in a horizontal plane perpendicular to the longitudinal axis X-X′ of the soot sensor, with this applying regardless of the radius R of the circular section of the protection shield 3.

As can be seen more particularly in FIGS. 2 to 4, the support body 2 advantageously presents means for indexing the position of the soot sensor 1 so that when the soot sensor is being put into place in an exhaust gas duct it is possible to ensure that the collection window in the protection shield 3 is oriented in the flow direction of the gas stream FG and thus on the downstream side of the stream. By way of example, these position indexing means may be constituted by a flat 4 formed in the cylindrical surface of the support body 2, the flat 4 being located in continuity with a continuous protection wall 31 of the protection shield 3 and in a face that is symmetrically opposite from the collection window 5 formed in the protection shield 3. This flat 4 may extend over some or all of the length of the support body 2 of the sensor, and it preferably extends over substantially 50% of its length, so as to provide at its base an abutment 41 against the cylindrical surface of the support body 2, this abutment 41 also serving to position the soot sensor 1 in depth while it is being inserted into an exhaust duct. Thus, by means of such a flat 4, the sensor may be positioned accurately both in depth and in angle in an exhaust duct provided with an insertion orifice of a shape complementary to the shape of the support body 2.

The protection shield 3 of the soot sensor 1 of the present invention has the function of protecting a planar probe 6 of any type, e.g. a capacitive or resistive probe 6 as shown in FIGS. 5A and 5B. The planar probe is fastened to the support body 2 by any appropriate means, in such a manner that the planar probe 6 extends inside the protection shield 3.

The planar probe 6 has a plane dielectric substrate 7 made of ceramic, preferably of alumina, with a first face 7 a (FIG. 5A), and two electrodes 8, 9 of conductive material, preferably gold or platinum, capable of withstanding the high temperature of motor vehicle exhaust gas, which is about 900° C. The two electrodes 8, are connected by electrical conductors to two electrical connection tracts 10, 11 enabling the contacts of the electrodes 8, 9 to be taken to the base of the substrate 7 to enable the planar probe 6 to be connected to the electronic control unit of the vehicle in charge of making electrical resistance measurements between the electrodes 8, 9 in order to monitor the concentration of soot in the exhaust stream of the vehicle.

The substrate 7, the electrodes 8, 9, their connectors and their tracks 10, 11 are covered in a dielectric material, e.g. glass or glass ceramic, with the exception of a top portion defining a soot collection zone 12 occupying a rectangle of length H and of width L. This collection zone 12 constitutes an active portion of the planar probe 6 where soot contained in the exhaust gas stream is collected in order to measure resistance between the electrodes 8, 9.

On a second face 7 b of the substrate 7, opposite from the first face 7 a and shown in FIG. 5B, the planar probe 6 has an electrical resistor 13. The resistor 13 is connected by electrical connectors to two tracks 14, 15 for connection to the electronic control unit of the vehicle. This second face 7 b of the probe is also covered in a dielectric material, e.g. glass or glass ceramic, in order to protect the resistor 13, which resistor is advantageously located symmetrically on the face 7 b relative to the electrodes 8, 9 and the collection zone 12 on the face 7 a. In a variant, the resistor 13 may also be integrated inside the substrate of the planar probe.

In known manner, the main function of the resistor 13 is to regenerate the planar probe 6 of the sensor 1 when the collection zone 12 and the gap between the electrodes 8, 9 have become clogged with soot, thereby establishing conductive bridges between the electrodes 8, 9. This regeneration is controlled electronically by the electronic control unit of the vehicle or of the sensor to which the planar probe 6 is connected by means of its tracks 10, 11 and 14, 15 when the resistance across the terminals of the electrodes 8, 9 saturates or drops below a determined threshold, indicating that the collection zone 12 has become clogged. The electronic control unit then causes the resistor 13 to heat up so as to burn off the soot that has become deposited between the electrodes 8, 9.

Another function of the resistor 13 is to make it possible to measure the temperature within the exhaust stream in which the soot sensor 1 is positioned.

The soot collection zone 12 may have various shapes and dimensions, e.g. depending on the size of the planar probe 6. By way of example, this width L preferably lies in the range about 3 millimeters (mm) to 4 mm. The height h of the collection window 5 of the protection shield 3 is then a function of the height H of the collection zone 12 of the planar probe 6. In preferred manner, in the context of the invention, the ratio of the height h of the collection window 5 to the length H of the collection zone 12 of the planar probe 6 then preferably lies in the range 0.5 to 3, and still more preferably in the range 1 to 2. It should be observed that the dimensioning of the planar probe 6 and of the protection shield 3 are selected in such a manner that the planar probe does not have any contact with the protection shield 3 so as to allow the gas stream that penetrates through the collection window 5 to flow inside the protection shield 3 and over each of the faces 7 a, 7 b of the planar probe 6.

The soot sensor 1 of the invention may be used in various configurations. In preferred manner it may serve in particular to diagnose the operating state of a motor vehicle particle filter, in particular for the purpose of determining whether it is cracked or intact. Under such circumstances, the soot sensor 1 is positioned in the exhaust duct of the vehicle downstream from the particle filter.

In another application, the soot sensor 1 may also be positioned upstream from the particle filter in order to determine its loading state for the purpose of optimizing stages of regenerating the particle filter and thereby minimize fuel consumption of the vehicle.

By using the protection shield 3 of the planar probe, the soot sensor 1 of the invention makes it possible to limit the negative consequences associated with the high speed and the high temperature of the exhaust gas stream from an internal combustion engine. The protection shield 3 serves firstly to protect the measurement planar probe 6 from direct impact of the stream and the soot particles it contains against the face 7 a and the electrodes 8, 9. It also protects the regenerating resistor 13 on the face 7 b of the planar probe 6 from a direct flow of the gas stream, thereby implying greater heating power for the resistor during stages of regenerating the planar probe 6.

The protection shield 3 deflects the exhaust gas stream FG upstream from the planar probe 6 and produces a turbulent flow of stream lines downstream from the probe, past the collection window 5 in the protection shield 3. This turbulent flow slows down the speed of the stream in the vicinity of the collection window 5, which in turn produces suction that tends to encourage the disturbed stream to penetrate into the shield and thus encourage soot deposition at slower speeds on the planar probe 6.

The place where the soot impacts against the collection zone 12 of the planar probe 6 determines the response of the sensor to soot. It is therefore essential for the soot to impact the face 7 a of the planar probe 6 in the collection zone 12 of the probe, regardless of the speed of the exhaust stream and always with proportions that are similar, i.e. with a ratio of impact speed to flow speed that is constant, in order to be insensitive to the speed of the stream. That is why, in preferred manner, the face 7 a and the collection zone 12 are situated facing the collection window 5. In other words, the face 7 a of the planar probe 6 faces towards the collection window 5.

Furthermore, the soot generated by an internal combustion engine can agglomerate and thus form agglomerations presenting diameters of several micrometers. If these agglomerations impact directly against the planar probe 6 of the soot sensor 1, they can saturate directly the electrodes 8, 9 of the planar probe, and then given their size, they can become separated and be carried away by the stream, thereby disturbing the effective measurement of the concentration of soot in the exhaust stream. It is therefore important to avoid collecting such agglomerations.

Because of the deflection produced by the shield, the agglomerations contained in the gas stream continue, after being deflected, to travel along a path that is substantially rectilinear in the exhaust stream downstream from the protection shield 3 of the soot sensor 1 of the invention because of their inertia, whereas non-agglomerated particles are subjected to the disturbance of the gas stream downstream from the sensor so they follow a disturbed flow path with vortices encouraging an effect whereby particles return towards the collection window 5 in the protection shield 3 and towards the planar probe 6 arranged inside it.

The protection shield 3 of the soot sensor of the invention also provides a large amount of mechanical protection to the planar probe 6 against potential metal and/or ceramic debris from the particle filter in the event of the filter failing.

Finally, the protection shield 3 protects the planar probe 6 from any liquid splashes that might lead to thermal shocks and breaks in the ceramic constituting the planar probe 6.

Consequently, the soot sensor 1 of the present invention serves not only to provide mechanical protection of the resistive planar probe 6 against impacts and the temperature of the exhaust stream, but also serves advantageously to select between soot particles so as to minimize the collection of agglomerated particles while in contrast encouraging the collection of fine particles, i.e. in practice, particles having a maximum dimension of less than 5 micrometers (μm). Furthermore, the soot sensor of the invention makes it easier to regenerate the sensor by burning off soot, even for gas streams at high speed and low temperature.

By disturbing the generated exhaust stream, the soot sensor 1 of the invention also enables soot particles to be deposited at reduced speed and in uniform manner on the planar probe 6, thereby encouraging a reliable response from the sensor 1 over time so as to obtain maximum insensitivity to particular conditions occurring in the exhaust stream.

In the invention, the above-described soot sensor 1 thus makes it possible to define and perform a new method of collecting soot for the purpose of measuring soot concentration with the help of a planar probe 6 of resistive type or even of capacitive type.

In the method, the gas stream upstream from the planar probe is deflected by means of a protection shield completely covering the planar probe on all of its faces and including a single open soot-collection window formed in a surface of the protection shield that is situated downstream from the planar probe in the gas stream, so as to:

-   -   protect the probe from a direct frontal impact against soot and         any debris that might be contained in the gas stream; and     -   generate a turbulent flow and reduce the speed of the gas flow         downstream from the planar probe in order to select particles as         a function of their size;     -   thereby giving rise to mechanical protection for the planar         probe and to a uniform deposit of soot on said planar probe.

While performing the method, soot is collected in the collection zone 12 of the first face 7 a of the planar probe 6 that is set back from the collection window 5 formed in the protection shield 3.

Care is preferably also taken to ensure that the soot collection zone 12 on the first face 7 a of the planar probe 6 is situated facing the collection window 5 formed in the protection shield 3.

Finally, care should also be taken to ensure that the soot sensor 1 is inserted in the gas stream in such a manner that the longitudinal axis X-X′ of the protection shield 3 extends perpendicularly to the flow direction of the gas stream and the planar probe 6 extends likewise longitudinally inside the shield 3 along its longitudinal axis X-X′.

FIGS. 8 and 9 show more particularly the large effect of the particle sorting obtained by the protection shield 3 of the soot sensor 1 of the invention.

Thus, as can be seen in FIG. 8, which reproduces various simulations performed for a sensor in accordance with the invention and provided with a protection shield 3 of semicylindrical shape placed in front of a planar probe 6 as described above in an exhaust gas stream FG containing various soot particles of size lying in the range 10 nm to 5 μm, it is found that for particle sizes in the range 10 nm to 500 nm the concentration of particles downstream from the planar probe 6 is very large, with a maximum stream disturbance causing soot to return towards the planar probe 6.

Conversely, for particles of larger size, in particular in the range 750 nm to 5 μm, the concentration of particles on the planar probe 6 and the stream disturbances downstream from the planar probe 6 are much more limited, and indeed non-existent for particles of 5 μm. The soot sensor 1 of the invention thus greatly enhances the collection of particles of small size and in particular particles having a size lying in the range several tens to several hundreds of nanometers, whereas larger particles that may be considered as being agglomerations of particles are not collected.

These observations are also confirmed in FIGS. 9A and 9B which respectively plots the concentration of particles as measured with the help of a soot sensor 1 of the invention as a function of the size of the particles for different given flow speeds of the exhaust stream, and as a function of the speed of the particles in the exhaust stream for different given sizes.

Thus, in FIG. 9A, it can be seen that overall, regardless of the flow speed of the stream in the range 10 meters per second (m/s) to 100 m/s, the concentration of particles collected drops very suddenly when the particles present a size greater than 100 nm.

This observation is also confirmed looking at FIG. 9B, which likewise shows that the concentration of particles of size greater than 500 nm decreases with increasing speed of the exhaust stream, while the concentration of particles of sizes lying in the range 10 nm to 100 nm does indeed vary with increasing speed, but does not drop significantly following a regular trend with increasing flow speed. 

1. A method of collecting soot in a gas stream, in particular an exhaust stream from an internal combustion engine, by means of a planar probe, preferably of resistive or capacitive type, positioned in the gas stream and having at least one face provided with electrodes connected to an electronic device for measuring soot concentration in the gas stream, the method being characterized in that the gas stream upstream from the planar probe is deflected by means of a protection shield completely covering the planar probe and having a single soot-collection window formed in a wall of the protection shield that is situated downstream from the planar probe in the gas stream, in such a manner as to: protect the planar probe from a direct frontal impact of soot and any debris contained in the gas stream; and generate a turbulent flow and reduce the speed of the gas stream downstream from the planar probe so as to select soot particles as a function of their size; thus resulting in mechanical protection of the planar probe and a uniform deposit of soot on said planar probe.
 2. A method according to claim 1, characterized in that the soot that is collected is selected by means of the protection shield so that the largest dimension of the collected soot particles is less than or equal to 5 μm.
 3. A method according to claim 1, characterized in that soot is collected in a collection zone on the first face of the planar probe facing the collection window formed in the protection shield.
 4. A method according to claim 1, characterized in that the protection shield is of right tubular shape having a longitudinal axis (X-X′), and of cross-section that presents a closed regular geometrical shape inscribed within a circle centered on the longitudinal axis of the protection shield.
 5. A method according to claim 4, characterized in that the longitudinal axis (X-X′) of the shield extends perpendicularly to the flow direction of the gas stream, and in that the planar probe extends longitudinally inside the protection shield along its longitudinal axis (X-X′).
 6. A soot sensor for measuring soot concentration in a gas stream, in particular an exhaust stream from an internal combustion engine, the sensor comprising a resistive type planar probe made up of a plane substrate of insulating material and having: on a first face at least two conductive electrodes separated from each other by a determined distance and each connected to an electrical connection track; and at least one resistor for regenerating the electrodes and for measuring the temperature of the gas stream, said resistor being connected at its terminals to two electrical connection tracks; the sensor being characterized in that it includes a protection shield covering the planar probe over all of the faces of its substrate and including a single open soot-collection window for collecting soot arranged in a wall of the protection shield situated downstream from the planar probe in the gas stream, the collection window enabling soot to be deposited on at least one collection zone on the first face of the planar probe when the sensor is positioned in a gas stream.
 7. A soot sensor according to claim 6, characterized in that the collection zone on the first face of the planar probe and the collection window arranged in the protection shield are arranged facing each other.
 8. A soot sensor according to claim 6, characterized in that the collection zone on the first face of the planar probe and the collection window arranged in the protection shield (3) are of rectangular shape, and the ratio of the length h of the collection zone to the height H of the collection window lies in the range 0.2 to 3, and preferably in the range 1 to
 2. 9. A sensor according to claim 6, characterized in that the protection shield is of right tubular shape presenting a longitudinal axis (X-X′) and of cross-section presenting a closed regular geometrical shape inscribed in a circle centered on the longitudinal axis along which the shield extends.
 10. A sensor according to claim 9, characterized in that the protection shield presents a cylindrical shape of circular cross-section, the planar probe being arranged in the inside space of the shield so as to extend along its longitudinal axis (X-X′).
 11. A sensor according to claim 10, characterized in that the collection window covers an angular sector lying in the range 45° to 180° in a horizontal plane perpendicular to the longitudinal axis (X-X′) of the protection shield.
 12. A sensor according to claim 6, characterized in that the collection window is subdivided into at least two openings by at least one reinforcing element extending between two opposite sides of the collection window.
 13. A sensor according to claim 6, characterized in that the planar probe does not make contact with the protection shield so as to enable the gas stream that penetrates via the collection window to flow inside the shield and over each of the faces of the probe.
 14. A sensor according to claim 6, characterized in that the regeneration resistor is positioned on a second face of the planar probe in a manner that is symmetrical to the soot collection zone on the first face of said planar probe relative to the longitudinal midplane of the probe.
 15. A sensor according to claim 6, characterized in that it includes a support body for the planar probe and a protection shield, the support body including indexing means for indexing the position of the sensor in a gas stream such that the collection window situated on the downstream side in the stream flow direction.
 16. A sensor according to claim 15, characterized in that the position indexing means comprise a flat on a cylindrical surface of the support body. 